1. Field of the Invention
The present invention relates generally to an apparatus and method for a power-saving mode operation, and in particular, to an apparatus and method for enabling a station to wake up periodically or non-periodically, to carry out data transmission and reception for a time, and then to transition to a sleep mode.
2. Description of the Related Art
A Wireless Local Area Network (WLAN), based on Institute of Electrical and Electronic Engineers (IEEE) 802.11, is an expansion of an existing wired LAN. It was developed for short-range communications for stationary users in restricted areas like indoor locations, densely populated downtowns, or university libraries. The WLAN offers Internet connectivity at a high rate of between 11 and 54 Mbps according to the IEEE standard. WLANs are spreading rapidly due to cost-effective Access Points (APs) or Access Routers (ARs) and their easy installation.
Saving energy is very crucial in a wireless communication system such as IEEE 802.11 because such a system is not AC-powered but relies on built-in power.
A conventional power-saving mode operation will be described with reference to FIG. 1. Referring to FIG. 1, upon wakeup, a second station (STA 2) sends a null frame to an AP to indicate the wakeup in step 101. In step 103, the AP replies with an ACKnowledgement (ACK) within a Short Inter Frame Space (SIFS). Then the AP sends a data frame to a first station (STA 1) according to its schedule in step 105. STA 1 sends an ACK for the received frame to the AP within the SWFS in step 107. In step 109, the AP sends a data frame to STA 2 according to its schedule. STA 2 then replies with an ACK within the SIFS in step 111.
When STA 2 has a frame to send to the AP, it sends the data frame to the AP in step 113 and the AP then replies with an ACK within the SIFS in step 115. In step 117, the AP sends a data frame to STA 2, as scheduled. STA 2 sends an ACK for the data frame to the AP within the SIFS in step 119. If STA 2 has not received data from the AP for a predetermined period of time, STA 2 sends a null frame with a Power Save (PS) bit set to ‘1’ in a header to the AP and then transitions to a sleep mode.
As described above, even though the power-saving station, STA 2 notifies the AP of its wakeup, the AP sends data frames as scheduled rather than sending data frames to the station with a priority. Hence, a long time is taken for the STA to receive buffered data from the AP. Since no data is received for a predetermined period of time of active-sleep mode transition, the wakeup time is highly unpredictable and much power is wasted. Moreover, as stations access each frame by Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA), transmission overhead is high. Considering that a decision criterion as to a wakeup time for an STA is yet to be specified, the conventional power-saving mode is not viable in practice.
IEEE 802.11 n-based frame aggregation under the standard performs well in terms of power consumption, although developed for the purpose of high throughput. Accordingly, a need exists for a method of solving the conventional problems by utilizing frame aggregation.